Hardware loops

ABSTRACT

In one embodiment, a programmable processor is configured to support a loop setup instruction. The loop setup instruction may be decoded and a zero offset loop may be detected from the loop setup instruction. The next instruction in the instruction stream may then be immediately issued as a first instruction in a loop. The loop setup instruction may also be used to detect a single instruction loop.

BACKGROUND

This invention relates to hardware loops implemented in a programmableprocessor.

In designing a programmable processor, such as a digital signalprocessing (DSP) system, two competing design goals are processing speedand power consumption. Conventional processors include a variety ofhardware designed to increase the speed at which software instructionsare executed. The additional hardware, however, typically increases thepower consumption of the processor.

One technique for increasing the speed of a programmable processor is a“hardware loop,” which may be dedicated hardware designed to expeditethe execution of software instructions within a loop construct. Hardwareloops may reduce the number of clock cycles used to execute a softwareloop by caching the instructions in local registers, thereby reducingthe need to fetch the same instruction from a memory device orinstruction cache a plurality of times.

Hardware loops introduce several challenges. These challenges includeavoiding penalties such as setup penalties or branch penalties. Setuppenalties are the loss of performance (usually an increase in processingtime) associated with setting up a hardware loop. Similarly, branchpenalties are the loss of performance (again, usually an increase inprocessing time) associated with a branch.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example of a pipelinedprogrammable processor according to an embodiment of the invention.

FIG. 2 is a block diagram illustrating an example execution pipeline forthe programmable processor in accordance with an embodiment of thepresent invention.

FIG. 3 is flow diagram illustrating exemplary timing of when earlyregisters may be loaded in accordance with an embodiment of the presentinvention.

FIG. 4 is a flow diagram illustrating how a loop setup instruction maybe used to determine early register values in accordance with anembodiment of the present invention.

FIG. 5 is a circuit block diagram illustrating one embodiment of ahardware loop unit.

FIG. 6 is a flow diagram illustrating a mode of operation involvingadjustment of early registers in accordance with the present invention.

FIG. 7 is a flow diagram illustrating a way of updating architecturalregister values in accordance with an embodiment of the presentinvention.

FIG. 8 is a flow diagram illustrating an alternative way of updating thearchitectural count register values in accordance with an embodiment ofthe present invention.

FIG. 9 is a flow diagram illustrating the timing advantages realized bythe use of early registers in accordance with an embodiment of thepresent invention.

FIG. 10 is a flow diagram illustrating an efficient way of adjusting anearly count value in accordance with an embodiment of the presentinvention.

FIG. 11 is a block diagram illustrating counters in a circuit inaccordance with an embodiment of the present invention.

FIG. 12 is a flow diagram illustrating a hybrid mode of operation inaccordance with an embodiment of the present invention.

FIG. 13 is a block diagram illustrating a hybrid circuit having countersonly in the first n−1 execution stages in accordance with an embodimentof the present invention.

FIG. 14 a block diagram illustrating a hardware loop unit in accordancewith an embodiment of the present invention.

FIG. 15 is a diagram illustrating a scenario in accordance with anembodiment of the present invention.

FIG. 16 is a flow diagram illustrating a scenario in accordance with anembodiment of the present invention.

FIG. 17 is another flow diagram illustrating a scenario in accordancewith an embodiment of the present invention.

FIG. 18 is a diagram illustrating another scenario in accordance with anembodiment of the present invention.

FIG. 19 is a flow diagram illustrating the other scenario.

FIG. 20 is a flow diagram illustrating both scenarios in accordance withan embodiment of the present invention.

FIG. 21 is a flow diagram illustrating a mode of operation for detectinga zero offset loop in accordance with an embodiment of the presentinvention.

FIG. 22 is a flow diagram illustrating a mode of operation for detectingand dealing with a single instruction zero offset loop in accordancewith an embodiment of the present invention.

FIGS. 23A-23C are block diagrams illustrating a number of hardware loopunits connected to one another in accordance with an embodiment of thepresent invention.

FIG. 24 is block diagram showing power saving circuitry in accordancewith an embodiment of the present invention.

DESCRIPTION

FIG. 1 is a block diagram illustrating a programmable processor 2arranged to support efficient hardware loops without significantlyincreasing power consumption.

In order to support hardware loops, a processor 2 may support a loopsetup instruction that initializes the hardware by setting entry andexit conditions for the loop. Entry and exit conditions may be definedby loop conditions: top, bottom and count. The top condition defines thefirst instruction (or top) of a loop. The bottom condition defines thelast instruction (or bottom) of a loop. And the count condition definesthe number of iterations of the loop.

Entry of a hardware loop may occur at the first “top match.” A top matchmay occur when the program counter (PC) is pointing to the topinstruction of a loop. Exit of a hardware loop may occur at the last“bottom match.” A bottom match may occur when the PC is pointing to thebottom instruction of a loop.

By initializing the count at the first top match and decrementing thecount at each bottom match, the hardware may keep track of when it hasencountered the last bottom match. In this manner, the loop conditionstop, bottom and count may define the entry and exit conditions of ahardware loop.

Processor 2 may include an execution pipeline 4 and a control unit 6.Control unit 6 may control the flow of instructions and/or data throughpipeline 4 during a clock cycle. For example, during the processing ofan instruction, control unit 6 may direct the various components of thepipeline to decode the instruction and correctly perform thecorresponding operation including, for example, writing the results backto memory.

Instructions may be loaded into a first stage of pipeline 4 andprocessed through subsequent stages. A stage may process concurrentlywith the other stages. Data may pass between the stages in pipeline 4during a cycle of the system. The results of an instruction may emergeat the end of the pipeline 4 in rapid succession.

Control unit 6 may include hardware loop unit 8 that, as describedbelow, may facilitate fast hardware loops without significantlyincreasing power consumption of processor 2.

FIG. 2 is a block diagram illustrating an example pipeline. A pipeline10 has multiple stages that may facilitate execution of multipleinstructions during a single clock cycle. In pipeline 10, an instructionmay enter the instruction fetch (IF) stage 12 during a first clockcycle. The instruction may then continue down the pipeline duringsubsequent clock cycles. Another instruction may enter the IF stage 12after a previous instruction has exited. Thus, typically anotherinstruction enters the IF stage 12 during a subsequent clock cycle andthen continues down the pipeline during subsequent clock cycles.Similarly, additional instructions enter the IF stage 12 duringsubsequent clock cycles respectively. The number of stages in thepipeline may define the number of instructions that the pipeline mayservice simultaneously.

The different stages of the pipeline may operate as follows.Instructions may be fetched during the IF stage 12 by a fetch unit 13and decoded from instruction registers 15 during the DEC stage 14.During the AC stage 18, data address generators 19 may calculate anymemory addresses used to perform the operation.

During the execution stages (EX 1-EX n) 22 and 28, execution units 23and 29 may perform specified operations such as, for example, adding ormultiplying two numbers. Execution units may contain specializedhardware for performing the operations including, for example, one ormore arithmetic logic units (ALU's), floating-point units (FPU) andbarrel shifters, although the scope of the present invention is notlimited in this respect. A variety of data may be applied to theexecution units such as the addresses generated by data addressgenerators, data retrieved from memory or data retrieved from dataregisters. During write back stage (WB) 30, the results may be writtento a memory location or data registers external to the pipeline or todata registers in the pipeline such as architectural registers 32. Thestages of pipeline 10 may include one or more storage circuits, such asa flip-flop, for storing data.

As mentioned above, processor 2 may support a loop setup instruction.The loop setup instruction may initialize the hardware loop by writingthe boundaries of the hardware loop (e.g. top and bottom) toarchitectural registers 32 in the pipeline. The loop setup instructionmay also initialize a count in architectural registers 32, indicatingthe number of times the loop is to be completed. In addition, the loopsetup instruction may define an offset, indicating the number ofinstructions that follow the loop setup instruction before the top ofthe loop is reached. After the hardware loop is initialized, thehardware loop may operate in the pipeline 10 until the exit condition ofthe loop has been satisfied (e.g. a bottom match with count equal tozero).

Architectural registers 32 are generally loaded once an instruction hascommitted, e.g., at the conclusion of the WB stage 30. Therefore, theentry and exit conditions stored in architecture registers 32 may not beupdated until several clock cycles have passed from when the loop setupinstruction enters pipeline 10. Because the entry and exit conditionsmay not be updated until several clock cycles have passed, restrictionson setting up hardware loops may exist. For example, if the firstinstruction in the loop enters pipeline 10 before the loop setupinstruction has committed, the architectural registers may not be set upto identify the instruction as part of a loop. Moreover, this problemmay increase as the depth of the pipeline increases.

In one embodiment, processor 2 may overcome these restrictions bymaintaining a set of early registers 34, 35 and 36 in the pipeline. Asshown in FIG. 2, early registers ETop 34 and EBot 35 may reside in thedecode stage while ECnt 36 may reside in AC stage.

Implementing a set of early registers 34, 35 and 36 may increaseprocessing speed of processor 2 by reducing or avoiding loop set uppenalties. As described above, several clock cycles may pass between thetime a loop setup instruction enters the pipeline and the time thearchitectural registers are written. However, the early registers may beloaded long before the loop setup instruction writes to thearchitectural registers. For this reason, implementing early registersmay reduce the time it takes to setup hardware loops.

The early registers may be speculative registers used to predict orspeculate the value of architectural registers. Unlike the architecturalregisters, the speculative registers may not be supported by thesystem's instruction set. Therefore, program code may not be used toaccess the speculative registers. For this reason, a programmer may notbe able to move data in or out of the speculative registers the same waythat he or she could with architectural registers.

Loading early registers may be done in several different ways. Forinstance, the early registers may be loaded simply as a result ofperforming a regular instruction register move to the architecturalregisters. In other words, the system may instruct the architecturalregisters to load the contents of some other register, and as a resultthe early registers may be updated. Yet another way to load theregisters is a “pop” from memory. In other words, the system may fetchthe data from memory, load the architectural registers with that data,and update the early registers.

The problem with regular register moves or pops, however, is that theymay introduce loop setup penalties. These penalties may occur becausethe system may stall the pipeline until the “moved” or “popped” data isavailable to be written. To avoid these penalties, a loop setupinstruction may be used to load the early registers before thearchitectural registers get written.

The following example illustrates the syntax for invoking an exemplaryloop setup machine instruction:

LSETUP (PC Relative Top, PC Relative Bottom) Counter=X

The PC Relative Top specifies the distance from the current instructionto the start of the loop (the Start Offset). The PC Relative Bottomspecifies the distance from the current instruction to the end of theloop (the End Offset). In addition, the Counter variable may specify acounter register and a loop count indicating the number of iterations inthe loop.

FIG. 3 is a flow diagram illustrating the timing when early registersget loaded in accordance with an embodiment of the present invention. Asdescribed, the loop setup instruction may contain loop conditions in theform of a count value, a top value, and a bottom value. Collectively,these three values may define entry and exit conditions of a hardwareloop.

The count value may represent the number of iterations that the loopwill make. Once the loop setup instruction enters AC (38), the countvalue may be written to the ECnt register (39). Initially writing to theECnt register may be done via a register move from data registerscontained in a different pipeline. In one mode of operation, the ECntregister may be written with data contained in PREG registers (see FIG.6) in the DAG pipeline.

The top and bottom values may indicate which instruction is the top ofthe loop, and which instruction is the bottom of the loop. The top andbottom values in the loop setup instruction, however, may be programcounter (PC) relative. Therefore, a calculation (40) in AC stage may beused to obtain the top and bottom values that will be written to ETop 34and EBot 35 registers respectively. After the loop setup instructionenters EX 1 (41), the top and bottom values may be written to the ETop34 and EBot 35 registers (42).

The ETop register 34 may be loaded to point to the first instruction ofthe loop (or top of the loop). The EBot register 35 may be loaded topoint to the last instruction of the loop (or bottom of a loop). TheECnt register 36 may be loaded to specify the number of times that thecircuit is to go through the loop. In one embodiment, ECnt 36 countsdownward, decrementing as the circuit goes through the loop.

FIG. 4 is a flow diagram illustrating one mode of operation of hardwareloop unit 8 when a loop setup instruction is received and processed bypipeline 10. According to one format, the loop setup instruction mayspecify several setup variables including a Start Offset (S-Offset) andan End Offset (E-Offset). The S-Offset may specify the distance in theinstruction stream from the loop setup instruction to the firstinstruction in the loop. Similarly, the E-Offset may specify thedistance in the instruction stream from the loop setup instruction tothe last instruction in the loop.

For instance, if the first instruction in the loop is the instructionimmediately following the loop setup instruction, then the S-Offsetwould be the width of the loop setup instruction. If, in the instructionstream, there is one instruction between the loop setup instruction andthe first instruction in the loop, then the S-Offset would be the widthof the loop setup instruction and the one instruction. Similarly, ifthere are two instructions between loop setup and the first instruction,then the S-Offset would be width of the loop setup instruction and thetwo instructions.

As shown in FIG. 4, the S-Offset and E-Offset are typically specified bya loop setup instruction (44). However, the loop-setup instructionspecifies the offsets relative to the program counter (PC). Therefore,the PC value must also be determined (45). The PC value and S-Offset maythen be used to calculate ETop register data (46). Moreover, the PCvalue and E-Offset may be used to calculate EBot register data (47).Once calculated, the early register data may be written to the earlyregisters (48).

Comparing FIG. 4 with FIG. 2 illustrates exemplary timing when writingthe ETop and EBot registers. Steps (44) and (45) may occur in DEC stage14. Calculations steps (46) and (47) may occur in AC stage 18.Therefore, the write step (48) may occur in EX 1 stage 22, although thescope of the invention is not limited in this respect.

Once loaded, the early registers may be used to set up hardware loops.FIG. 5 is a block diagram illustrating one embodiment of a hardware loopunit 8 connected to an instruction fetch (IF) unit 50 and a decoder unit52 of pipeline 10. In one embodiment, the early registers may be used todetect a loop in the stream of instructions 57. Loop hardware 54 is thenloaded with one or more loop instructions. Once loaded, the loopinstructions may be issued again and again from loop hardware. Thus, ifthe early registers detect an instruction loop, then one or more of theloop instructions may be fetched only once by IF unit 50, and thenissued repeatedly from hardware loop unit 8.

Implementing early registers may introduce several challenges. Forinstance, one challenge introduced by the use of early registers arisesbecause the early register data may need to be adjusted. If the pipelinemust service an event, the pipeline's current operations may need to beterminated. If this termination occurs after an early register has beenwritten, but before its respective architectural register has beenwritten, then the early register data may need to be adjusted. In otherwords, because the early registers are written early, a termination inthe pipeline before the loop has committed may require an adjustment ofthe early register data.

FIG. 6 is a flow diagram illustrating a mode of operation involvingadjustment of early registers. In particular, FIG. 6 illustrates thetiming involved in adjusting ETop EBot, ECnt registers.

As shown in FIG. 6, the early registers may get written (76). In oneembodiment of FIG. 6, the early registers are specifically the ETop,EBot, and ECnt registers (as shown in FIG. 2). If a terminate in thepipeline (80) occurs before the architectural registers have beenwritten (82), then the unaborted instructions may be drained from thepipeline (83), and the early registers may be adjusted by writing thedata contained in their respective architectural counterparts (84).

The mode of operation illustrated in FIG. 6 may be particularly usefulwhen dealing with back-to-back hardware loops. If, for instance, asecond loop is terminated before it commits, a first loop might stillneed to execute its remaining instructions in the pipeline. Theadjustment technique in FIG. 6 provides a way to deal with thistransition by adjusting early register values accordingly.

FIG. 7 illustrates a way of updating the architectural register valuewith the Early Count (ECnt) data. Data may be written to an ECntRegister (90) in any manner described above. The ECnt register data maythen be sent down the pipeline (92) so that it gets written to thearchitectural count register (93) during WB stage. Then, in everysubsequent iteration of the loop (94) the ECnt Register data may bedecremented (96) and re-sent down the pipeline (92). This process maycontinue until the loop has finished its last iteration (98).

FIG. 8 illustrates an alternative way of updating the architecturalcount register. Rather than sending the ECnt data down the pipelineafter every iteration of the loop, the mode of operation illustrated inFIG. 8 sends the ECnt data once. Then, in subsequent iterations of theloop, a single valid decrement bit may be sent to adjust thearchitectural count register.

In FIG. 8, data may be written to an ECnt Register (90) in any mannerdescribed above. The ECnt register data may then be sent down thepipeline (92) so that it gets written to the architectural countregister (93) during WB stage. In subsequent iterations of the loop(94), a single valid decrement bit may be sent (102) to decrement thearchitectural count register (104). This process may continue until theloop has finished its last iteration (98).

FIG. 9 is a flow diagram illustrating the timing advantages realized bythe use of early registers. As shown in FIG. 9, loop entry/exitconditions may be loaded into a set of early registers (120). Theseearly entry/exit conditions may then be detected (122) before a loopsetup instruction has committed (124).

Adjusting the ECnt register may present additional challenges.Adjustment of the ECnt register may be done any time there is atermination of the pipeline prior to a loop being finished. This may bebecause valid instructions may still be in the pipeline and these validinstructions might adjust the architectural count register value oncethey commit.

One way of correcting the ECnt register data following a termination isto allow the instructions before the termination to commit and thenwrite the ECnt register with its architectural counterpart. However,this may add an additional penalty if the branch penalty is less thanthe time it takes to drain the pipeline.

FIG. 10 illustrates an efficient way of adjusting an early count value.As shown, after a loop setup instruction enters AC stage (180), theearly count register may be written (182). At this point, the ECnt datamay be sent down the pipeline (184). If a termination (185) occurs,unaborted instructions may be drained (186) and the ECnt register may bewritten with the data in its architectural counterpart (187).

At each bottom match (190) a valid decrement bit may be sent down thepipeline to increment counters (192). When an instruction that caused avalid decrement bit to be sent exits each pipe stage (194), a validdecrement bit may be removed from the counter at that exited pipe stage(196). In other words, the counters may be incremented when a valid bitis sent (e.g. at 192) and an individual counter may then be decremented(e.g. at 196) when the instruction that caused the valid decrement bitto be sent exits the pipe stage.

If a termination occurs in the pipeline (198), the early count registermay be adjusted (188) by the number of valid decrement bits in thecounter at the termination stage. This adjustment value may also berepresented in the respective counter of every pipe stage that resideslater in the pipeline than the stage where the termination occurred. Thesteps in FIG. 10 may repeat themselves until all loop instructions haveexited the pipeline (200).

The ECnt register may be decremented at every bottom match (e.g. forevery pass through the loop). However, rather than propagate the wholeECnt value down the pipeline after every bottom match, only a singlevalid decrement bit may be sent down the pipeline. In this manner, ahardware reduction may be realized in the DSP system. The set ofcounters may monitor valid decrement bits and the associatedinstructions that caused those bits to be sent. As such, the set ofcounters may keep a running tab on differences between an early countregister and its architectural counterpart.

FIG. 11 is a block diagram illustrating counters in a circuit. As shown,counters may be maintained at every stage of a pipeline after DEC (210,211, 212 and 213). Therefore, a single valid decrement bit may bepropagated down the pipeline to account for the difference between theearly count register and its architectural counterpart. The counter maybe connected to the input of a multiplexer 220. Multiplexer 220 maydetermine which counter, if any, is used to adjust the early countregister 36.

In other embodiments, a termination may not occur at certain stages, ormay be restricted when a certain instruction resides in a certain stage.In such embodiments, the circuit hardware may be reduced by notimplementing counters in the stages where a termination is restricted.

The width of counters 210, 211, 212 and 213 may be altered as needed.For instance, counter 210 may be a one-bit counter and 211 may be atwo-bit counter. The minimal depth of counters 212 and 213 may depend onthe number of execution stages n in the pipeline. Counter 210 may be aone-bit counter because it always has a value of either 1 or zero. Forinstance, it has a value of one if a bottom match instruction iscurrently in AC. When the instruction leaves AC, the counter returns tozero.

Counter 211 and the next counter (e.g. for stage EX 2) may be two-bitcounters, although the scope of the present invention is not limited inthis respect. This is because counter 211 (e.g. the EX 1 counter) mayhave a maximum value of two, the EX 2 counter may have a maximum valueof three. The EX 3 counter has a maximum value of four, hence, it mayneed three bits. Similarly, the next three counters (e.g. EX 4 to EX 6)may be three-bit counters capable of representing between five to sevenvalues respectively.

The counter width for each stage may be determined by considering themaximum value that the counter may need to hold. This may correspond tothe case where the loop is a single instruction loop. Thus, the depth ofthe counters may simply correspond to the number of stages between thecounter and AC. Again, however, the size may also depend on whether atermination may occur in a particular stage; so some stages may not needcounters.

Another mode of operation is a hybrid way of adjusting the early countregister. The counters residing at the selected stages of the pipelinemay be used to adjust the early count register if the penalty associatedwith draining the pipeline is greater than the branch penalty. However,if the drain penalty is less than or equal to the branch penalty, thenthe instructions in the pipeline may be allowed to commit so that theearly count register may be adjusted from its architectural counterpart.System operation may be chosen depending on where in the pipeline thetermination occurred. In one case, a pipeline having 3 execute stagesmay be drained if the termination occurs in EX3 stage or WB stage; butthe pipeline may not be drained and the early counter may be adjusted ifthe termination occurs before the EX 3 stage.

FIG. 12 is a flow diagram illustrating a hybrid mode of operation. Asshown, a counter may adjust an early count value (228) if a termination(230) occurs before the n^(th) stage. However, if termination (230)occurs after the n^(th) stage then the instructions in the pipeline maybe allowed to flow through the pipeline and commit (232). The earlyregisters may then be updated with the data in their architecturalcounterparts (233). The variable n may define the point at whichallowing instructions to flow through the pipeline takes an amount oftime less than or equal to the branch penalty (234).

FIG. 13 illustrates a hybrid circuit having counters in the first n−1execution stages. The circuit may allow the pipeline to execute itsinstructions following a termination if an instruction has entered then^(th) execution stage. However, the circuit may adjust an early countregister following a termination if an instruction has not entered thenth execution stage. Again, the variable n may define the point at whichallowing instructions to flow through the pipeline takes an amount oftime less than or equal to the branch penalty. In other cases, thevariable n may reside much earlier in the pipeline (e.g. where thebranch penalty is larger).

Additional loop hardware, such as loop registers and loop buffers, mayfacilitate fast hardware loops. Loop hardware as described herein, mayprovide several advantages. In some implementations, loop hardware mayhide branch penalties. In addition, loop hardware may save power byremoving the need to access memory devices such as cache or SRAM forevery iteration of a loop. Furthermore, the loop hardware describedbelow may have very few programming restrictions. Moreover, loophardware may remove alignment restrictions that might otherwise exist inprogram memory.

Referring again to FIG. 1, illustrated is a programmable processor 2having an execution pipeline 4 and a control unit 6. Control unit 6 mayinclude hardware loop unit 8.

Hardware loop unit 8 may include one or more sets of loop buffersconnected in series. In addition, the hardware loop unit may include oneor more single instruction loop registers. A collection of loop buffersand/or loop registers may collectively be loop hardware. This collectionmay facilitate fast hardware loops.

Referring again to FIG. 5, illustrated is a block diagram showing anembodiment of a hardware loop unit 8 connected to an instruction fetchunit 50 and a decoder unit 52 of pipeline 10. Instruction fetch unit 50may provide one of a number of inputs to the hardware loop unit 8.Hardware loop unit 8 may comprise a multiplexer 56 connected to loophardware 54. Loop hardware 54 may contain one or more sets of loopbuffers and/or one or more loop registers. Moreover, the sets of loopbuffers may include a defined number of buffers connected in series.Each set of loop buffers may have an associated loop register.

FIG. 14 is another block diagram illustrating one embodiment of ahardware loop unit 8. Again, hardware loop unit 8 may be connected to aninstruction fetch unit 50 and a decoder (not shown).

Hardware loop unit 8 may include a multiplexer 260 connected to loophardware. Loop hardware may include a set of loop buffers 271, 272, 273and 274 connected in series. These buffers may provide input to anothermultiplexer 280. Multiplexers 260 and 280, in turn, may provide inputsto multiplexer 290. Output of multiplexer 290 may be connected to a loopregister 296, which may be connected back to the input of multiplexer260. The output of hardware loop unit 8 may provide input to a decoder.

Multiplexer 260 may have a plurality of inputs. These may include aninput from a register (e.g. an emulator instruction register) and aninput from the fetch unit 50. In addition, multiplexer 260 may have aninput from the loop instruction register and an input from one or moreother hardware loop units.

Exemplary FIG. 14 illustrates loop hardware comprised of a 4-deep set ofloop buffers and a single instruction register. However, any number ofbuffers and/or instruction registers could be used in other embodiments.

In exemplary modes of operation, loop buffers are implemented in twodifferent scenarios. In one scenario, every instruction in the loop fitsin the loop buffer. In another scenario, every instruction in the loopdoes not fit in the loop buffer. Each scenario is discussed in turn.

FIG. 15 is a diagram illustrating one scenario. As shown, a 4-deep setof loop buffers may be loaded with a three-instruction loop (I2-I4). Assuch, one of the loop buffers and the VTop register may not be loadedwith any data. Instructions I2-I4 may be executed again and again, untilthe exit condition of the loop is met. As shown, the exit condition maybe checked every time the I4 instruction is issued from the loop buffer.

FIG. 16 is a flow diagram illustrating one scenario. As shown,instructions may be issued (300), instructions may be loaded into a setof loop buffers (302), and the memory device may be stalled (304).Stalling the memory may provide power savings to the DSP system. Afterloading the instructions into the set of buffers (302), the instructionsmay be subsequently issued from the set of buffers (306). Theinstructions may be issued over and over again until the exit conditionof the loop is met (308).

FIG. 17 is another flow diagram illustrating one scenario. As shown,instructions may be issued (310), instructions may be loaded into a setof loop buffers (314), a memory device may be stalled (316), and a loopcount may be initialized (312). As long as the exit condition of theloop is not met (318), the loop count may be decremented (320) and theinstructions may be issued from a set of loop buffers (322). The memorydevice may be stalled until the loop count reaches a defined number X(316). Moreover, X may be defined such that the next instruction afterthe loop may be immediately ready in the fetch unit when the loop hasfinished. In this manner, a loop exit penalty may be avoided.

In other embodiments, a loop count is initialized (312) to zero.Adjustment (320) would then increment the loop count rather thandecrement as shown in FIG. 17. Still other embodiments will becomeapparent in light of FIGS. 15-17.

FIG. 18 is a diagram illustrating another scenario. As shown, a 4-deepset of loop-buffers may be loaded with four instructions of a loop(I2-I5). In addition, a VTop register may be loaded with the address ofthe next instruction in the loop. VTop may be an address register thatresides in the pipeline (see FIG. 2, VTop register 37). Duringsubsequent iterations of the loop, instructions I2-I5 may be executedwhile I6 is being fetched. I6 may then be immediately ready forexecution after I5 has been executed. The exit condition may be checkedduring iterations of the loop.

FIG. 19 is a flow diagram illustrating the scenario of FIG. 18. Asshown, a first subset of instructions may be loaded into a set ofbuffers (330), as the first subset of instructions is issued (332).Next, the starting address of the second subset of instructions may beloaded into a register (334) and the second subset may be issued (336).During subsequent iterations of the loop (the number of iterations beingdefined by the exit condition (338)), the first subset of instructionsmay be issued from the set of buffers (340) while the second subset isbeing fetched (342). The second subset may then be issued (344).

A mode of operation in accordance with FIG. 19 may realize severaladvantages. For instance, every time the first subset is issued from aset of buffers, power may be saved if the memory device is not firing.In addition, fetching a second subset while a first subset is issued mayminimize loop penalties. In one embodiment, the loop buffer may have adepth that corresponds to at least the branch penalty. Thus, the penaltyassociated with fetching the second subset may be hidden behind theissuance of the first subset of instructions. This may increase thespeed of the DSP system. In other embodiments, however, where powersaving is a more important design feature (e.g. for circuits implementedin battery powered devices), deeper buffers may be more useful.

The first subset of instructions may be the number of instructions thatfit in the set of loop buffers. The second subset of instructions may bethe next instruction in the loop following those loaded in the buffers.Alternatively, the second subset may be a plurality of instructions thatfollow those loaded in the buffers.

FIG. 20 is a more general flow diagram illustrating the scenariosdescribed above. As shown, a first subset of instructions may be loadedinto a set of buffers (350) and issued (352). If all instructions of theloop fit into the loop buffer (354), then a memory device may be stalled(355) and the instructions may be continuously issued from the set ofbuffers (356) until the exit condition is met (358).

If all the instructions of the loop do not fit into the loop buffer(354), then a second subset of instructions may be issued (360) and thestarting address of those instructions may be loaded into a register(362). On subsequent iterations of the loop (as defined by the exitcondition (364)), the first subset of instructions may be issued fromthe set of buffers (366) while the second subset is fetched from memory(368). The second subset may then be issued (370).

In the scenario of FIG. 18, a request for the address pointed by VTopmay be made every time there is a bottom match. In this manner, a firstsubset of instructions may be dispatched while fetching a second subsetof instructions.

By the time the first subset of instructions in the loop has beendispatched, the cache/memory may have already been accessed and analignment buffer primed with the second subset of instructions. Thealignment buffer may hold multiple instructions (depending on the widthof each instruction). If the second subset includes a plurality ofinstructions, the plurality may be aligned in the alignment buffer withthe first instruction of the second subset being an instructionassociated with a VTop register.

Operation may further include requesting the next instruction followingthat associated with VTop while the instruction associated with VTop isdispatched. For example, in one mode of operation, the instructionassociated with VTop is a 64-bit instruction. As that 64-bit instructionis dispatched, the next 64-bit instruction (as defined by alignment inmemory) may be fetched. By repeating this over and over again, a DSPsystem may dispatch instructions in a fast and efficient manner.

Prior art programmable processor systems that implement hardware loopsmay require many restrictions in the operating code. However, it ishighly desirable to provide for hardware loops that have very fewprogramming restrictions. In one embodiment, a system in accordance withthe present invention may have only one programming restriction. Thesingle restriction may be that the system cannot have a branch on a loopbottom.

Fast hardware loops may have situations where a loop buffer calls forinvalidation. These situations might include invalidation where a loopbuffer has been populated with a conditional branch. Thus, in one modeof operation, a set of loop buffers may be invalidated if one of thefirst n instructions in a loop is a conditional branch. The variable nmay define the depth of the set of loop buffers

Another situation where a loop buffer may require invalidation is whenan event is received while a loop buffer is being loaded. In addition, aloop buffer may require invalidation following a C_SYNC forself-modifying code. Thus, other modes of operation involve invalidatinga loop buffer when either of these two situations arise.

Zero offset loops may introduce additional challenges. Offsets may referto the distance between the loop setup instruction and the first or lastinstruction in the loop. In prior art systems, offsets are necessary tofacilitate setup of hardware loops or to account for programmingexceptions in the system. Facilitating zero offset loops, however, ishighly advantageous because it may reduce processing time.

Even in a system that implements early registers and loop buffers, zerooffset loops may raise challenges. For instance, in the system describedabove, by the time the early registers are written (e.g., in Ex 1) thefirst instruction of a zero offset loop may already be in AC. Thus,since the beginning of the loop exits DEC even before Etop and Ebot getwritten, the circuit may effectively “miss” a top match.

Special modes of operation and circuit arrangements have been designedto detect and deal with zero offset loops. In one mode of operation thestart offset (S-offset) in a loop setup instruction is compared to aknown value to detect zero offset. In this manner, early detection of azero offset loop is facilitated. In a particular embodiment, theS-offset is compared to 4, where 4 refers to the four bytes of a 32 bitinstruction. Once detected, a zero offset loop may be set up even beforethe early registers get written.

FIG. 21 is a flow diagram that illustrates a mode of operation fordetecting a zero offset loop. As shown, a loop setup instruction may bedecoded (380). The s-offset may then be compared to a defined number(382) (the defined number corresponding to the width of the loop setupinstruction). In this manner, a zero offset loop may be detected evenbefore the early registers get written. If zero offset loop is detected(384), the next n instructions in the loop may be issued and loaded intoa loop buffer (386) as described above.

FIG. 22 is a flow diagram that illustrates a mode of operation fordetecting and dealing with the special case of a single instruction zerooffset loop. As shown, a loop setup instruction may be decoded (388). Ifzero offset is detected (390), and the Start Offset equals the EndOffset (392), then a single instruction loop may be immediatelyperformed (394). Zero offset may be detected when the s-offset equalsthe width of the loop setup instruction. A single instruction loop maybe detected when the s-offset and e-offset are the same.

Special hardware may facilitate setting up and executing a singleinstruction loop as quickly as possible. Referring again to FIG. 14, theloop register 296 may be used for this special case. Thus, step (394)may be performed by loading loop hardware (such as loop register 296)with the single instruction, and repeatedly issuing that instruction outof hardware loop unit 8 until the exit condition of the singleinstruction loop is met.

A single instruction loop may be detected and then performed by holdingthe instruction in decode in a special loop instruction register 296(see FIG. 14). In addition, memory devices may not be accessed andeverything upstream may be stalled to save power. Moreover, since thesingle instruction loop may be issued out of loop instruction register296, the instruction fetch unit 50 may be free to fetch the nextinstruction (e.g. from a cache). Thus, when the single instruction loophas finished execution the next instruction may already be in theinstruction fetch unit 50, resulting in zero penalty loop exit.

In summary, zero offset hardware loops may be broken into three cases.In the first case, a zero offset single instruction loop may beimmediately issued out of decode via a loop instruction register. In thesecond case, a zero offset n-instruction loop may be immediatelydetected and loaded completely into an n-deep set of loop buffers. Andin the third case, a zero offset loop may be immediately detected andthe first n-instructions of that loop loaded into an n-deep set of loopbuffers. The next instruction may then be associated with a VTopregister.

In one embodiment, loop hardware as described herein, may be implementedin a pipeline. This may be much more advantageous than implementingbuffers in a cache.

In one mode of operation, loop instructions may be stored in loophardware such that the instructions are aligned to an instructionboundary. This may be more advantageous than aligning instructions to anaddress boundary. Once aligned to an instruction boundary in loophardware, no alignment of instructions may be necessary when they aresubsequently issued from the loop hardware.

To align instructions to an instruction boundary, a set of instructionsmay be loaded in an order of execution into the loop hardware and theaddress of the next instruction may be stored in an address register. Inaddition, a second set of instructions may be loaded in order ofexecution into a memory device. Loading instructions in an order ofexecution into loop hardware may comprise loading n sequentialinstructions into n buffers, where n is a positive integer. Moreover,the n sequential instructions may be any sequential instructionsincluding the first n instructions of a loop or the last n instructionsof a loop.

In other embodiments (as shown in FIGS. 23A-23C), a number of hardwareloop units may be implemented. The output of any of the number ofhardware loop units may be connected to the respective input of one ormore other hardware loop units. In this manner, a second hardware loopunit may be loaded via the output of a first hardware loop unit. Again,this may provide power savings insofar as a memory device is not firedup to load the second hardware loop unit.

In some cases, a plurality of loop units may be used to service nestedloops (or coincident loops). An inner hardware loop unit may hold one ormore instructions that are aligned in an outer hardware loop unit.Moreover, the inner hardware loop unit may be loaded by sending thealigned instructions from the outer hardware loop unit to the input ofthe inner hardware loop unit.

In still other embodiments, an independent plurality of hardware loopunits may be implemented. However, if two or more independent hardwareloop units are used in the same DSP system, a restriction may be used.For instance, when both loops have the same bottom instruction, one ofthe loops may be defined as the outer loop. Without this restriction thecircuit might encounter two bottom matches and not know which one todecrement.

Yet another mode of operation involves power management of hardwareloops. When hardware loops are enabled, an ETop register may be comparedto the PC to account for each top match. Likewise, an EBot register maybe compared to the PC to account for each bottom match. However, whenhardware loops are disabled, any switching in the comparator circuitsmay be merely a waste of power. Therefore, it may be advantageous todisable bottom match and top match comparator circuits when a hardwareloop is disabled. By choosing inputs to a comparator to disable thecomparator when the hardware loop is disabled, power may be conserved.

FIG. 24 is an exemplary embodiment showing power saving circuitry. ETopregister 400 and EBot register 402 respectively provide one of a numberof inputs to multiplexers 404 and 406. The output of multiplexers 404and 406 may be inputs to comparators 408 and 410. The other input tocomparators 408 and 410 may come from the output of multiplexer 412. Theinputs of multiplexer 412 may come from the program counter 414 and aloop disable signal 416 from a control unit. Multiplexers 404, 406 and412 may be arranged such that when hardware loops are disabled, theoutput of multiplexer 412 is different from the output of multiplexers404 and 406. This may ensure that no switching occurs in comparators 408or 410 when the hardware loops are disabled.

In one embodiment, the same loop disable signal 416 may be sent tomultiplexers 404, 406 and 412. However, an inverter (not shown) mayinvert a bit before multiplexer 412 receives the signal. In this manner,the output of multiplexer 412 may be different than that of multiplexers404 and 406 when a loop disable signal is sent to the circuit.

FIG. 24 also illustrates an exemplary circuit for incrementing an ECntregister 418. On each pass of the loop, multiplexer 420 may decrementthe ECnt register. However, if adjustment is necessary, the adjustmentssignal (e.g. the clobber count 422) may adjust the ECnt accordingly.Once the loop has finished its last iteration, comparator 424 may send asignal 425 so indicating.

FIG. 24 also illustrates how bottom matches and top matches may bedetected. When comparator 410 detects that the program counter 412 andETop register have the same value, a Top Match signal 428 may be sent.When comparator 408 detects that the program counter and the EBotregister have the same value, a Bottom Match signal 430 may be sent.

Finally, FIG. 24 also illustrates a VTop register 432 implemented in apipeline. As described, herein, VTop may be an address register holdingthe address of a next instruction following a plurality of instructionsloaded into loop hardware (not shown).

Various embodiments of the invention have been described. For example,numerous hardware loop techniques have been described for implementationwithin a processor. The processor may be implemented in a variety ofsystems including general purpose computing systems, digital processingsystems, laptop computers, personal digital assistants (PDA's) andcellular phones. In this context, the hardware loops discussed above maybe readily used to increase processing speed without significantlyincreasing power consumption. In such a system, the processor may becoupled to a memory device, such as a FLASH memory device or a staticrandom access memory (SRAM) that stores an operating system and othersoftware applications. These and other embodiments are within the scopeof the following claims.

What is claimed is:
 1. A method comprising: decoding a loop-setupinstruction; detecting a zero offset loop from the loop setupinstruction by comparing a start offset value in the loop setupinstruction to a width of the loop setup instruction, the zero offsetloop having zero instructions between the loop setup instruction and afirst instruction in the loop; and issuing a next instruction followingthe loop-setup instruction as a first instruction in a the zero offsetloop.
 2. A method comprising: decoding a loop-setup instruction;detecting a zero offset loop from the loop setup instruction; andissuing a next instruction following the loop-setup instruction as afirst instruction in a loop, wherein the zero offset loop is detectedbefore loop conditions of the loop are written to a set of registers. 3.The method as in claim 1, the method further comprising: loading loophardware with instructions, the first instruction loaded into the loophardware being the next instruction following the loop setupinstruction.
 4. The method as in claim 3, the method further comprising:issuing at least some of the instructions from the loop hardware duringa subsequent iteration of the loop.
 5. The method as in claim 1, themethod further comprising: detecting a single instruction loop from theloop setup instruction.
 6. The method as in claim 5, the method furthercomprising: loading a loop register with the next instruction followingthe loop setup instruction.
 7. The method as in claim 6, the methodfurther comprising: issuing the next instruction from the loop registerduring subsequent iterations of the loop.
 8. The method as in claim 1,wherein the zero offset is detected by comparing a start offset value inthe loop setup instruction to a defined number.
 9. An apparatuscomprising: a control unit adapted to detect a zero offset loop from aloop setup instruction by comparing a start offset value in the loopsetup instruction to a width of the loop setup instruction, the zerooffset loop having zero instructions between the loop setup instructionand a first instruction in the loop; and loop hardware to store aninstruction from the zero offset loop.
 10. The apparatus as in claim 9,wherein loop hardware includes at least two loop buffers coupled inparallel, and the control unit is adapted to issue instructions and loadat least some of the instructions into the at least two loop buffers,the first instruction loaded into the buffers being a next instructionfollowing the loop setup instruction.
 11. The apparatus as in claim 10,wherein the control unit is adapted to issue instructions from the loopbuffers for a subsequent iteration of the loop.
 12. The apparatus as inclaim 9, wherein the control unit is adapted to detect a singleinstruction loop.
 13. The apparatus as in claim 12, wherein loophardware includes a single instruction loop register, and the controlunit is adapted to issue a next instruction following the loop setupinstruction and load the next instruction into the single instructionloop register after detecting a zero offset single instruction loop. 14.The apparatus as in claim 13, wherein the control unit is adapted toissue the next instruction from the single instruction loop register ona subsequent iteration of the loop.
 15. A system comprising: a SRAMmemory device; and a processor coupled to the memory device, wherein theprocessor includes a control unit adapted to: decode a loop-setupinstruction; detect a zero offset loop from the loop setup instructionby comparing a start offset value in the loop setup instruction to awidth of the loop setup instruction, the zero offset loop having zeroinstructions between the loop setup instruction and a first instructionin the loop; and issue a next instruction following the loop-setupinstruction as a first instruction in a loop.
 16. A system comprising: aSRAM memory device; and a processor coupled to the memory device,wherein the processor includes a control unit adapted to: decode aloop-setup instruction; detect a zero offset loop from the loop setupinstruction; and issue a next instruction following the loop-setupinstruction as a first instruction in a loop, wherein the control unitis adapted to detect the zero offset loop before loop conditions of theloop get written to a set of registers.
 17. The system as in claim 15,wherein the control unit is adapted to load the loop hardware withinstructions, the first instruction loaded into the loop hardware beingthe next instruction following the loop setup instruction.
 18. Thesystem as in claim 17, wherein the control unit is adapted to issue atleast some of the instructions from the loop hardware during subsequentiterations of the loop.
 19. The system as in claim 15, wherein thecontrol unit is adapted to detect a single instruction loop from theloop setup instruction.
 20. The system as in claim 19, the systemfurther comprising a loop register, wherein the control unit is adaptedto load the loop register with the next instruction after detecting asingle instruction zero offset loop.
 21. The system as in claimed 20,wherein the control unit is adapted to issue the next instruction fromthe loop register during a subsequent iteration of the loop.
 22. Thesystem as in claim 15, wherein the control unit is adapted to detect thezero offset by comparing a start offset value in the loop setupinstruction to a defined number.
 23. A method comprising: decoding aloop-setup instruction; detecting from the loop setup instruction thatthe top and bottom of a loop are pointing to the same instruction; andimmediately after detecting the top and bottom of a loop are pointing tothe same instruction, performing a single instruction loop in a decodestage of a pipeline.
 24. The method of claim 23, wherein performing asingle instruction loop comprises: issuing the single instruction out ofthe decode stage; loading the single instruction into a loop register;and reissuing the instruction from the loop register.
 25. The method asin claim 24, the method further comprising: fetching a secondinstruction that follows the single instruction loop.
 26. The method asin claim 25, the method further comprising: issuing the secondinstruction after the first instruction has been reissued from the loopregister a defined number of times.
 27. The method of claim 23, whereinthe loop setup instruction includes a start offset and an end offset,and wherein detecting from the loop setup instruction that the top andbottom of the loop are pointing to the same instruction comprisesdetermining that the start offset is equal to the end offset.
 28. Themethod of claim 23, further comprising: detecting zero offset from theloop setup instruction, and issuing the next instruction immediatelyfollowing the loop setup instruction as the first instruction in a zerooffset single instruction loop.
 29. The method of claim 23, whereindetecting zero offset comprises comparing a start offset to a definednumber.
 30. The method of claim 29, wherein detecting zero offsetcomprises comparing a start offset to the width of the loop setupinstruction.
 31. An apparatus comprising: a control unit adapted todetect from a loop setup instruction that the top and bottom of a loopare pointing to the same instruction and immediately after detecting thetop and bottom of a loop are pointing to the same instruction,performing a single instruction loop starting in a decode stage of apipeline; and loop hardware to store the single instruction.
 32. Theapparatus as in claim 31, wherein the loop hardware includes at leastone single instruction loop register, and wherein the control unit isadapted to load an instruction into the single instruction loop registerafter detecting from the loop setup instruction that the top and bottomof the loop are pointing to the same instruction.
 33. The apparatus asin claim 32, wherein the control unit is adapted to issue theinstruction from the single instruction loop register on iterations ofthe loop.
 34. The apparatus as in claim 31, wherein the control unit isadapted to detect a zero offset loop from the loop setup instruction,and issue a next instruction immediately following the loop setupinstruction as the first instruction in a zero offset single instructionloop.
 35. The apparatus as in claim 34, wherein detecting a zero offsetloop comprises comparing a start offset to a defined number.
 36. Theapparatus as in claim 35, wherein the defined number corresponds to thewidth of the loop setup instruction.
 37. A system comprising: an SRAMmemory device; and a processor coupled to the memory device, wherein theprocessor includes a control unit adapted to: decode a loop-setupinstruction; detect from the loop setup instruction that the top andbottom of a loop are pointing to the same instruction; and immediatelyafter detecting the top and bottom of a loop are pointing to the sameinstruction, perform a single instruction loop starting in a decodestage of a pipeline.
 38. The system of claim 37, the system furthercomprising a single instruction loop register, wherein performing asingle instruction loop comprises: issuing a loop instruction out of adecode stage; loading the loop instruction into a loop register; andreissuing the loop instruction from the loop register.
 39. The system asin claim 37, wherein the control unit is further adapted to detect azero offset loop from the loop setup instruction, and issue a nextinstruction immediately following the loop setup instruction as thefirst instruction in a zero offset single instruction loop.
 40. Thesystem as in claim 39, wherein detecting a zero offset loop comprisescomparing a start offset to a defined number.
 41. The system as in claim40, wherein the defined number corresponds to the width of the loopsetup instruction.
 42. The system as in claim 38, wherein the loopinstruction is a first instruction, the control unit being furtheradapted to: fetch a second instruction that follows the singleinstruction loop.
 43. The system as in claim 42, the control unit beingfurther adapted to: issue the second instruction after the firstinstruction has been reissuing from the loop register a defined numberof times.